There are two general approaches for implementing current control for driving a DC brushless motor. The first is pulse width modulation, in which a driving voltage is modulated by a square wave. The duty cycle of the pulse width modulation signal determines the average voltage applied to the motor, which voltage determines the amount of current delivered to the motor. The other approach for implementing current control is known as linear current control. In this approach, an analog voltage input signal is provided which is proportional to the current to be delivered to the motor. The motor control circuitry processes the input signal and adjusts the level of current delivered by the power circuitry as a function of the input signal.
In order to efficiently drive a polyphase motor (e g., a three phase DC brushless motor), the excitation current in the motor phases should be aligned with the Back Electro Motive Force (BEMF) generated by the three phases. A common approach for aligning the excitation current and BEMF is the use of a phase-locked loop (PLL). The PLL adjusts the phase and frequency of the commutation so that the BEMF of an undriven (tri-stated) winding passes through zero in the center of the appropriate commutation state. This scheme works well when the shape of the commutation waveform includes an undriven region, as in a conventional six-state sequence. In addition to the undesirable acoustic noise, this step-function tri-stating of the undriven motor phases, together with the step-function driving waveform can produce torque ripple in the motor. The torque ripple usually results in unevenness in the motor rotation, which also excites resonances in the motor, causing undesirable acoustic noise.
Another aspect of motor control relates to speed regulation. The PLL can be used for speed regulation. For example, during each electrical cycle of the motor, one of the three motor phases will float for a short amount of time, during which the phase voltage is expected to pass through a known value. If the detected BEMF includes errors, such as when conventional sinusoidal current shaping is being used, for example, the accuracy of the BEMF can be reduced due to such errors. For example, a small amount of current present in the undriven phase during phase detection (e.g., due to flyback current through one of the parasitic diodes in the output stage of the motor driver) can introduce an error into the PLL proportional to the magnitude of current present. In addition to adversely affecting the accuracy of the BEMF, such error also tends to manifest itself as torque ripple during motor operation.